This invention relates to an electric discharge machine in which a machining gap or machining area is calculated from the rise time of a pulse voltage applied between the electrodes, to stabilize and optimize the machining operation.
FIG. 11 shows the arrangement of a conventional electric discharge machine of the type where metal or semi-metal powder material is mixed in a machining solution. In FIG. 11, reference numeral 1 designates an electrode; 2, a machining tank; 3, a workpiece; 4, a machining solution; 5, a smoothing circuit for smoothing interelectrode voltage; 6, a numerical control unit; 7, a servo amplifier for driving an actuator according to instructions provided by the numerical control unit 6; 8, an electric motor driven by the servo amplifier 7; 9, a slider to which the electrode 1 is fixedly secured, the slider 9 being movable in the Z-axis direction by the motor 8; and 10, a machine power source.
It is well known in the art that, in an electric discharge machine of this type, by mixing a powder of predetermined material of about 10 to 40 .mu.m in grain size with a machining solution to a mixing density of about 20 g/l, the mechanical characteristics of the surface of the electrode or workpiece such as corrosion resistance and wear resistance can be improved. That is, the metal surface treatment can be achieved by electric discharging as well as by electric discharge machining to machine and remove a portion of the metal workpiece. Powder materials of this type are semi-metals such as silicon, zirconium, tantalum, tungsten carbide, zirconium boride, and their compounds. A technique of forming a surface layer for a workpiece by using a machining solution containing such semi-metal powder is being developed. This technique will greatly increase the range of application of electric discharge machining.
The operation of the conventional electric discharge machine shown in FIG. 11 will be described.
The electrode 1 is confronted with the workpiece 3 with an interelectrode gap G in the machining tank 2 filled with the machining solution 4 containing powder material. The machining power source 10 comprises: a DC source E; a switching element SW for controlling the application of machining current; a current limiting resistor R; and an oscillator OSC for controlling the on-off operation of the switching element SW. A pulse current I is supplied between the electrode 1 and the workpiece 3. The pulse current I is: EQU I=(E-Vg)/R
where Vg is the interelectrode voltage.
The interelectrode voltage Vg is 20 to 30 V during arc discharge, and it is zero (0) volt when the electrode touches the workpiece. Furthermore, the interelectrode voltage Vg is E when no arc discharge takes place, and it is 0 V when the switching element SW is off. Hence, if the interelectrode voltage Vg is detected, and smoothed with the smoothing circuit 5, then the machining gap can be controlled according to the smoothed voltage. That is, when the machining gap is large, it is rather difficult to induce discharges, and the smoothed voltage Vs is high. When the machining gap is small, then discharges are induced with ease, and the smoothed voltage Vs is low. The smoothed voltage Vs is compared with a reference voltage Vr. In accordance with the difference between those voltages, the numerical control unit 6 applies an axial movement instruction to the servo amplifier 7. In response to the instruction, servo amplifier 7 drives the motor 8 thereby to move the slider 9 together with the electrode vertically. Thus, a motor servo mechanism comprising the motor 8 and the slider 9 maintains the machining gap G substantially unchanged.
There is available an electric discharge machine of the type where the machining solution contains no powder material. This conventional electric discharge machine is fundamentally equal in arrangement to the electric discharge machine shown in FIG. 11, although the former is different from the latter in that the machining solution has no powder material, and accordingly the former is fundamentally the same in operation as the latter.
As is apparent from the above description, in the conventional electric discharge machine, a general method of determining whether or not the machining condition is satisfactory is to detect the interelectrode voltage Vg. When the interelectrode voltage is low, the interelectrode impedance is low. The causes for this are, for instance, short-circuiting, continuous arc discharging, and presence of metal powder or sludge in the machining gap.
In the case where the machining solution contains powder material, the machining gap is several times as large as in the case where the machining solution contains no powder material. However, it has been confirmed through experiments that the machining gap depends greatly on the density and grain size of the powder therein. The electrode and the workpiece form a capacitor, the capacitance of which (hereinafter referred to as "the interelectrode capacitance", when applicable) greatly affects the machined surface roughness. In an electric discharge machining operation using a machining solution mixed with powder material, the machining gap is increased to decrease the interelectrode capacitance, to thereby improve the machined surface roughness.
Therefore, in the case when the powder density is decreased by consumption or local precipitation of the powder material, the machining gap is decreased greatly, as a result of which the interelectrode capacitance is increased, thus lowering the machined surface roughness. In the case where the machining solution contains powder material, the frequency of induction of unwanted arc discharges increases greatly with decrease of the machining gap, as a result of which the workpiece may be damaged greatly.
An electric discharge machining operation using a machining solution containing no powder material suffers from the following difficulty: If an abnormal arc discharge occurs, which is most serious in an electric discharge machining operation, carbon is formed by thermal decomposition of the machining solution, as a result of which electric discharges are induced between the carbon thus formed and the workpiece as if the interelectrode impedance were increased. Hence, it is impossible to detect from the smoothed voltage whether or not the interelectrode condition is acceptable.
The control of the discharge gap, employing the above- described smoothed voltage, suffers from the following difficulties: In the case when a large quantity of sludge is present in the discharge gap, secondary electric discharges take place frequently, so that the smoothed voltage is decreased. Hence, although the machining gap is large, it is detected as if it were small, as a result of which the machining gap is erroneously increased. Furthermore, in the case where the pause time is changed with the reference voltage Vr held constant, the machining gap is also changed. Accordingly, with automatic pause control, the machining gap is not secured, thus adversely affecting the machining accuracy.
In order to eliminate the above-described difficulties accompanying a conventional electric discharge machine, for instance Published Examined Japanese Patent Application No. 58252/1986 or 58254/1986 has proposed a method of estimating the machining gap from the difference between the present position and the most advanced position of the electrode. However, this method is still disadvantageous in that the most advanced position itself includes an error on the order of several tens of micrometers, and, when protrusions are formed locally by arcs, the measurement accuracy is greatly lowered.
Published Unexamined Japanese Patent Application No. 82127/1981 has disclosed a method in which high frequency voltage is applied to a machining gap, and the length of the machining gap is measured from the variation of the current flowing therein. However, this method is also disadvantageous in that it is necessary to additionally provide a high frequency source, and the measurement accuracy is not so high because the machining gap length is measured while the change in resonance condition is being detected.
Furthermore, the conventional electric discharge machine suffers from the following difficulties: If machining electrical conditions and reciprocation conditions are not suitably set for the machining area, then discharge concentration takes place to consume the electrode abnormally or produce arcs abnormally. Therefore, before a machining operation is started, it is necessary to calculate the approximate value of the electrode area (or machining area) thereby to suitably determine operating conditions for the electrode area thus calculated. In an actual machining operation, generally the machining area changes as the operation advances, and therefore it is necessary to write a program so that the machining conditions are changed with the advance of the machining operation. However, even if the operating conditions are changed in this manner, problems to be solved are involved as follows: In the case where the initial machining area is considerably small as in the case when electric discharge is started with a rib-shaped or spear-shaped electrode, and is abruptly increased as the machining operation advances, or in the case where, because the electrode is intricate in configuration, it is difficulty to estimate the change in the machining area with the advance of the machining operation; the program for changing the operating conditions is necessarily intricate, and it is rather difficult to maintain the machining conditions most suitable at all times. Thus, in the electric discharge machining operation with the above-described unique electrode, the machining characteristics such as machining speed and machining accuracy are greatly lowered.